Britain's international development secretary on Wednesday warned that up to 400,000 children could die through starvation if urgent action is not taken to help Somalia and its neighbours in the Horn of Africa.

Andrew Mitchell's grim warning came as he saw the devastating impact of famine and conflict on the country during a visit to a feeding centre and a camp for internally displaced people in the Somali capital, Mogadishu.

"I came to Mogadishu today to see first-hand how Britain can best help people that have been hit by this devastating famine," said Mitchell, the first British minister to visit Mogadishu in 18 years. "The stark fact is that in southern Somalia the situation is deteriorating by the day. We could face deaths on a similar scale to those seen in 1991-92 if we do not act urgently now. This is a race against time. Evidence of malnutrition is not just in the camps and feeding centres but on every street corner."

As Mitchell visited Mogadishu, Britain announced an extra £25m ($41m) in emergency aid for Somalia. The new package will go through Unicef, allowing the UN organisation for children to double the number of children it is reaching in its supplementary programme.

Somalia faces a severe humanitarian crisis and the worst food security crisis in Africa since its famine 20 years ago.

At least 3.7 million people – about half of Somalia's population – need food, and around 3.2 million people are in extreme need of immediate, lifesaving aid because of drought and years of conflict. In neighbouring Ethiopia, the UN refugee agency is reporting that 10 Somali children under the age of five are dying every day of hunger-related causes in Kobe refugee camp.

The Department for International Development (DfID) said its £25m children's package will provide up to 192,000 people with two months of supplementary rations, and supplies to vaccinate at least 800,000 children against measles, plus 300,000 with polio vaccines, vitamin A and deworming.

Mitchell urged other countries to step up their aid effort as the UN warns that more regions of Somalia will be hit by famine in the coming months.

Relief agencies estimate $2.48bn is required across the Horn. So far, $1.32bn has been raised – just over 50%. The UK, which helped sound the alarm by announcing aid to Ethiopia last month, has been lobbying the international community to send aid to the region.

"Other countries must also maintain and increase their support at this crucial stage," said Mitchell. "Or we risk seeing a whole generation of people decimated by starvation and disease – and further instability across the region."

The UK government has given £119m towards the UN appeal for east Africa. France has given £25m, Germany £25m and Italy just £3m. The African Union has so far pledged only $500,000 for the aid effort, and most key governments have pledged even less or nothing at all.

Meanwhile, the Organisation of the Islamic Conference (OIC) countries pledged $350m in aid for Somalia at an emergency summit in Istanbul.

"All in all we have secured $350m in pledges," said the OIC secretary general, Ekmeleddin Ihsanoglu, at the end of the summit. "We hope to raise the commitments to $500m in a very short time."

Earlier, the Turkish prime minister, Recep Tayyip Erdogan, urged rich Muslim countries to help, saying they bore some responsibility for the crisis for ignoring the poverty in the country.

Erdogan told a dozen foreign ministers and other officials that Islam dictates "that you do not go to bed full if your neighbour is hungry. If we had fulfilled our responsibilities, would our brethren nation Somalia be in this situation?" he asked. "This is not only a test for the Somali people, it is a test for all humanity."

Erdogan, is scheduled to go with his family to Somalia on Thursday. Accompanying him will be his foreign minister, Ahmet Davutoglu, who will also take his family. Davutoglu will go on to South Africa and Ethiopia.

Few foreign leaders visit Somalia because it is so dangerous, but Erdogan has said it is "impossible for us to be spectators to the human tragedy in Africa".

Al-Shabaab, the Islamist insurgents, have pulled out of the capital, Mogadishu, but security remains precarious with the threat of hit-and-run attacks and suicide bombings. The last leader to visit Somalia was the Ugandan president, Yoweri Museveni, last November. Ugandan troops form the backbone of the African Union peacekeeping force in Somalia, which props up the weak transitional government.